Popping sound coming from drivers side floor

[Dan P.]

I just got a new to me 2011 2500HD duramax with 99k miles on it. Just here recently every time I put the truck in park and it rolls until the transmission catches I hear a pretty loud popping sound from somewhere under the dash in the floor of the drivers seat. Everything works fine as far as I can tell, and I’m fairly certain it’s inside the truck. I can feel it in the brake pedal and sometimes I think I can in the steering wheel. It doesn’t do it in a turn or anytime the truck is moving in drive or reverse. Only does it when put in park and let go of the brake. I have no clue what it is. Thanks!